The invention was originally made whilst we were considering the production of vermiculite, and it is convenient to introduce it in that context. Vermiculite is mined in a hydrated state where it consists of thin sheets of mineral with associated water. It can be expanded, or exfoliated, to a volume 5 to 10 to 20 times its original size by heating the mineral. There is a large global industry in vermiculite—perhaps 600,000 tonnes are mined a year. Uses include fluid absorption—such as water absorption (e.g. in horticulture), friction products (such as brake pads), thermal insulation, refractory products, building construction boards and materials, in paints, and various other uses.
There are various grades of vermiculite depending upon the size of the particles being expanded. These range through: large, medium, fine, super fine, and micron (V4 to V0 in the USA). A place to reference these sizes is www.vermiculite.org/properties.htm
Expanded vermiculite is expensive to transport—it is low weight and high volume. Nevertheless, expanded vermiculite is transported across the world. The website gives average diameters of the expoliated vermiculite particles as:
Large8 mmMedium4 mmFine2 mmSuperfine1 mmMicron0.5 mm
The traditional way of exfoliating vermiculite is to use a large hydrocarbon-fueled kiln or furnace and add unexpanded vermiculite at the top, have the vermiculite pass through a heating zone in the kiln, and have expanded vermiculite fall out at the end of the kiln. This uses a lot of energy—perhaps 500-1000 kilowatt hours per ton. The kiln and associated equipment takes up a lot of space and it is an expensive capital piece of equipment. It needs to be kept running at all times otherwise the refractory lining of the kiln can crack and be damaged. The exfoliating vermiculite comes out at many hundred degrees centigrade and needs to stand in piles to cool down before it can be bagged. The vermiculite passing through the kiln rubs against itself and this often causes break up of the product to some degree. Indeed, dust extraction, and hot air handling, apparatus can also be a major part of an industrial vermiculite production kiln/furnace, and itself can use many hundreds of kilowatt hours of power, and be very expensive.
The vermiculite is heated to approximately 600° C. If it is heated for too long it can “cook”, and talc is produced instead of exfoliated vermiculite through removal of structural hydroxyl groups. A typical yield is about 85% (by mass), i.e. the mass of the expanded useful vermiculite is about 85% of the mass of the unexpanded vermiculite. Different furnaces or kilns with different temperatures and dwell times in the heating zones of the furnaces or kilns, are used to produce vermiculite of different grades/sizes.
There have been proposals to use microwaves to exfoliate minerals, such as vermiculite. However, none of them have really worked very well. Some involve adding hydrogen peroxide or ammonia to the vermiculite, and then applying microwaves. This is because microwaving it without pre-treatment has been found not to work effectively. For example this approach is given by U.S. Pat. No. 3,758,415.
Also published is PCT patent application WO 2006/127025 by Sklyarevich and Shevelev. This teaches the use of microwave radiation to expand perlite (a very different mineral structure to vermiculite). It suggests using 83 GHz microwave energy with a energy density of around 5 kilowatts per square centimeter of the propagating wave in free space, and a total power of around 13 kilowatt hours, with a production speed of about 30 grams per second.
It has been especially problematic to use microwaves to exfoliate fine grades of vermiculite, and it has not been demonstrated to use microwave exfoliation for any grade commercially at commercial volumes.